The present invention relates to an apparatus and method for use in channelizing telephone transmission equipment, and in particular, for identifying an interrupt from a channel unit in a PCM multiplexed system without requiring an individual interrupt. An interrupt provides a channel unit with the ability to request service from a central processing unit in the system.
The primary purpose of a PCM digital multiplexer system is the conversion of voice frequency signals to digital signals using standard sampling and encoding techniques and the multiplexing of 23 additional digitized samples into the T1 serial transmission format. Within the format the T1 serial bit stream are 24 eight bit locations commonly referred to as timeslots into which the encoded data can be placed. The normal approach to assigning these timeslots to the individual channels has been on a fixed physical slot to timeslot basis. In other words, each physical slot is identified to a particular timeslot.
In addition to the requirement of being able to assign any one of 24 available timeslots to the channel unit, each of the channel units must have the ability to request service from a central processing unit in the form of an interrupt signal. This interrupt signal would be used to identify to the central processing unit certain states or activities which are happening on each of the channel units customer interface leads. A particular requirement which led to this invention was the detection of a test voltage on the central office interface leads and the identification of this state to the central processing unit for further action.
At the present time there are two known methods used to select channel units for transmitting and receiving data from the serial bit stream at the respective timeslot. One known carrier system utilizes individual select leads from a common module which decodes the 24 states of a channel counter. This approach, although simple from a channel unit perspective results in a large number of common equipment pinouts and backplane connections. Other known systems perform the selection process in a similar manner with the decoding process performed on the channel unit instead of the common equipment. This approach results in less pinouts for the common equipment but more pinouts for the channel units. This approach to channel unit selection was previously sufficient for these non microprocessor hardware oriented carrier systems.
With the evolution of PCM carrier systems and a number of different timeslot assignment sequences, there is a requirement to provide the ability to re-assign physical slots to different timeslots within the T1 format in order to be backward compatible with the older systems. This requirement was addressed in prior art systems by altering the counting sequence of the channel counter only. This provided the ability to address other sequences in a simple manner. Future interoffice and digital loop carrier systems must not only provide this capability but also provide for a total random assignment of timeslots in order to address future formats and services as necessary.
An additional requirement is that a central processing unit must be able to communicate with individual channel units for various reasons. It was this requirement which led to the development of the present invention.